Digital images can be generated directly from scenes by digital capture devices, such as digital still or video cameras, or by scanning an image captured by a photographic negative or slide film, or by various other means. Whatever the form of capture, most digital images are ultimately intended for display either by printed hardcopy, projection or electronic viewing device. In order to provide the most pleasing display, it is necessary that the color and/or brightness of the displayed image be adjusted according to the subject matter of the scene.
In color photographic printers making prints from film negatives, various methods for determining amounts of exposure have been known and practically employed. A well-known printing system in which the printing light source intensity is adjusted during red, green and blue exposures to levels which will normalize the resulting integrated transmittances to a near-neutral color balance, i.e., “gray”, is based on U.S. Pat. No. 2,571,697 issued Oct. 16, 1951 to Evans. This printing system produces satisfactory results from a large majority of the negatives of a given type of film. It has also been known in the art to adjust the rate of correction for red, green and blue exposures based on a linear combination of the red, green and blue large area transmission densities (LATD) of the original to be printed. Since the above-described conventional printing systems are based on the integrated transmission measurements conducted over the whole area of the original, the prints obtained are not always satisfactory. For instance, if the background of the principal subject matter is primarily red (red curtain or furniture), green (green grass or foliage) or blue (blue sky or water), color correction based only on the aforesaid LATD system is unsatisfactory. This problem is known as “color failure”. Further, if the background of the principal subject matter is of particularly high or low brightness, the conventional correction based on the integrated transmission density does not give satisfactory results. For example, when the principal subject matter has been photographed with a back light or in a spotlight, conventional correction will give unsatisfactory results. This is known as “density failure” or “brightness failure”.
It has also been known in the prior art to determine the exposure in a color printer based on the measured densities of divided areas of color originals in which the entire area of the original is divided into upper and lower, right and left, and central and peripheral sections. The exposure is determined based on a combination of the LATD and the densities of the divided areas. In this system, the yield of satisfactory prints is somewhat raised. However, since the density of the principal subject matter is not accurately measured in this system, the correction is not always effected in the most desirable manner.
It is also known in the art that color failures can be substantially reduced by the use of the subject failure suppression technique described in the journal article “Modern Exposure Determination for Customizing Photofinishing Printer Response”, E. Goll, D. Hill and W. Severin, Journal of Applied Photographic Engineering, Vol 5, No 2, 1979. For color negative film printing systems, it is further known that the performance of the subject failure suppression technique is improved by determination of an exposure-level-dependent gray estimate for a particular length of film as disclosed in U.S. Pat. No. 5,959,720 issued Sep. 2, 1999 to Kwon et al.
Further, in looking at printed color photographs, it is well known that most people are concerned about the faces of the figures when present in the scene content. Therefore, in printers, it is desirable that the faces of the figures be printed in a good condition. An exposure controlled to obtain a good skin color and density can increase the yield of satisfactory prints.
It is known in the prior art, as in U.S. Pat. No. 4,203,671 issued May 20, 1980 to Takahashi et al., to print color originals based on the skin color areas when the originals contain over a certain number of points of skin color. In order to carry out this method, it is necessary first to detect skin color in the color original. Under the method of U.S. Pat. No. 4,203,671 (referenced above), a skin color area is defined as one whose red, green and blue densities fall within an ellipse when plotted in a two-dimensional coordinate system or within an ellipsoid when plotted in a three-dimensional coordinate system, the axes of which represent the red, green and blue densities or combinations of the densities of red, green and blue. When the measured color is contained in the predetermined ellipse or ellipsoid, the color is assumed to be skin. The predetermined ellipse or ellipsoid is constructed by measuring the color attributes of identified skin points in a number of color negatives.
U.S. Pat. No. 5,781,276 issued Jul. 14, 1998 to Zahn et al., also discloses a method for using points of skin color in determination of printing exposure amounts. This method also first requires the detection of points of skin color in the original, and also accomplishes this by determining whether a point falls within a predetermined color space. The predetermined color space is constructed by measuring the color compositions of identified skin points. The method relies on further logic to distinguish skin points from non-skin points.
For a series of original images, these methods may lead to unsatisfactory results for the following reasons:
First, the predetermined color space regions used to identify skin points can be unnecessarily large in order to accommodate the possible variations in skin color due to differences in capture illuminants, variability in film and photographic processing chemistry, leading to increased probabilities of false identification of skin points.
Second, the previously described predetermined color space regions used to identify skin points do not explicitly consider the probabilities of non-skin points falling into the same regions of color spaces, leading to increased probabilities of false identification of skin points.
Third, the use of predetermined regions constant for all images may not optimally discriminate skin points and non-skin points under variations in color and density that may occur in image capture systems.
Fourth, determination of exposure amounts solely on the basis of skin points may ignore other scene elements that are important to final print quality.
There is a need therefore, for an improved method of identifying skin colored points and an associated method of adjusting the image brightness that contributes to more desirable quality in the final image.